Dunn view is suitable for the evaluation of alpha angle in terms of the reproducibility with the maximum radial a angle. There was adverse correlation between radial alpha angle and signal intensity by MRI in younger and older age group, which is possibly influenced by the difference of pathological changes in subchondral bone.
Especially in cam-type FAI, alpha angle is an important parameter commonly evaluated by plain-X ray, however, 3-dimensional (3D) evaluation is essential for better prehension of cam morphology. Recent development of 3D modalities such as computed tomography or magnetic resonance imaging (MRI) with 3.0 Tesla specification may potentially contributes to this concern. In addition, quantitative evaluation by signal intensity of MRI that reflects the pathological changes in subchondral bone may have some correlation with the size of cam morphology, i.e., the radial alpha angle. In this study, we investigated the radial alpha angle using 3D MRI, comparing with radiographic alpha angle and signal intensity. The purposes of this study are therefore to compare the alpha angle between radial MRI and plain X-ray, and to reveal the correlation between radial alpha angle and signal intensity of MRI in cases with FAI and borderline developmental dysplasia of the hip (BDDH) with cam morphology.
A total of 43 hips of 39 patients (male 19; female 20) underwent arthroscopic surgery between October 2013 and May 2018 are enrolled in this retrospective observational study. The subjects included two groups of FAI with 22 joints (mean age: 41) and BDDH with 20 joints (mean age: 43). FAI is defined with the central edge (CE) angle greater than 25° in A-P view of the hip joint and alpha angle greater than 55° in cross table lateral view or 45° Dunn view. BDDH is defined with the CE angle between 20° and 25°in A-P view of the hip joint.
We compared maximum alpha angle in radial MRI to alpha angles in 3 different radiographic postures: A-P view, cross-table lateral view and 45°Dunn view. For the measurement of a angle, we adopted the method previously described by Bouma et al. Signal intensity in cam lesionis was evauated and compensated by the individual value.
In comparion with radiograph and radial MRI, 45°Dunn view showed the smallest discrepancy in both FAI and BDDH. Regarding to the correlation between the maximum radial alpha angle and radiographic alpha angle, cross-table lateral view in BDDH showed the strongest correlation (r=0.9, p<0.01). In all cases, correlation between alpha angle and signal intensity in radial MRI was not recognized. However, when considered in the patients with 30 years or younger (younger age group), weak negative correlation between a angle and signal intensity in radial MRI was recognized (r=-0.36, p=0.021). To the contrary, in the patients of 50 years or older (older age group), weak positive correction between alpha angle and signal intensity in radial MRI was recognized (r=0.33, p=0.008).
Discussion And Conclusion
The discrepancy between the maximum radial alpha angle by MRI and radiographic alpha angle by plain X-ray was the smallest in Dunn view, however, the correlation between them itself was the strongest in cross-table lateral view. There was adverse correlation between radial alpha angle and signal intensity by MRI in younger and older age group, which is possibly influenced by the difference of pathological changes in subchondral bone.